Wireless local area networks are becoming more commonplace as the use of portable computers, such as “laptop,” “notebook,” and “pen” computers become increasingly common in office environments and other locations. In most conventional wireless local area networks, a number of access point base stations form a cellular network for communicating with wireless mobile stations or other mobile devices. Each access point base station is typically connected to a network server, such as part of an ethernet or other network infrastructure. Any messages transmitted as wireless communication signals are first transmitted to an access point base station instead of transmitted along wireless stations. This type of centralized wireless communication using cells provides control over communications along existing wireless mobile devices. Typically, the wireless communication signals are a spread spectrum communications signal, for example, a direct sequence spread spectrum signal or a frequency hopping spread spectrum signal.
Although wireless local area networks are becoming more commonplace in offices and similar environments, most wireless local area networks do not provide the capability of determining the location of a wireless mobile device or other signal source operating in the wireless LAN environment. Although some wireless LAN systems provide for signal strength analysis of spread spectrum signals to determine location, none of them provide an accurate means of determining the location of a mobile device operative within the wireless infrastructure defined by access point base stations.
Various types of wireless local area network systems that use location systems of the assignee are disclosed in commonly assigned U.S. Patent Publication No. 2002/0080759 and U.S. Pat. Nos. 6,892,054 and 6,987,744, the disclosures which are hereby incorporated by reference in their entirety. Much of the disclosure in these incorporated by reference patents and published patent application rely on differential time-of-arrival (TOA) applications that receive a transmitted signal at multiple physical locations corresponding to a receiver to locate a tag transmitter. The clocks could be synchronized so that differences in the indicated time-of-arrival closely approximate the actual time-of-arrival difference. Commonly assigned U.S. Patent Publication No. 2007/0176749 filed Jan. 30, 2007 by the same inventor, and the disclosure which is hereby incorporated by reference in its entirety, discloses a location system used in a wireless local area network that determines differences in signal arrival times, for example, when receivers are unsynchronized.
Time-of-arrival measurements made with respect to independent (randomly related) clocks contained in non-synchronized receivers can be accurately related to each other if the receivers observe signals from sources at known locations. A WLAN signal source can be placed in a known fixed location that propagates directly to all participating receivers. Each receiver can measure the time-of-arrival for transmissions from the reference source and communicate them to a central clock tracking service, which could be hardware or software implemented. Knowing how much time is required for the reference source's signal to propagate to each receiver, the tracking service can evaluate what time each receiver clock indicated when the source transmitted. By observing reference signals sufficiently often, the tracking service can determine a continuous relationship between the independent clocks. This relationship can then be used to accurately relate time-of-arrival measurements for signals arriving from unknown locations.
Some locations systems support the 802.11 WiFi system that use the Receive Signal Strength Indication (RSSI) of transmitted signals from mobile stations. These WiFi networks use access points that are generally less expensive than knitting active RTLS systems together. WiFi access points operate in conjunction with WiFi based tags. Typically, location systems will use either one of the WiFi access points for RSSI processing or a WLAN time differential of arrival (TDOA) processing with various access points and tag transmitters that conform with the ISO 24730 real-time location system (RTLS) standards. There are some tag transceivers that use a dual mode of operation in which the tag transceiver can operate in one of a selected WiFi configuration for RSSI processing or the ISO 24730 mode for time differential of arrival processing. The RSSI processing system uses a locate map, but it would be advantageous to update the locate map with the accuracy of different location points such as applied from the WELAN time of arrival processing to allow non-dual mode mobile stations can be located.